As disclosed in Japanese Laid-Open Patent Publication No. 5-44434, a typical exhaust purifying apparatus applied to an internal combustion engine on a vehicle includes an exhaust purification catalyst located in an exhaust system. The exhaust purification catalyst functions to trap particulate matter (PM) and nitrogen oxides (NOx) contained in exhaust gas.
Such an exhaust purifying apparatus estimates the amount of particulate matter accumulated in an exhaust purification catalyst based on the operation state of an engine. When the amount of the accumulated particulate matter is no less than a permissible value, the apparatus performs heating control to regenerate the catalyst, the performance of which has been degraded due to clogging of particulate matter. In the heating control, the apparatus supplies fuel to the exhaust purification catalyst to heat the catalyst, and uses the heat to burn and remove particulate matter accumulated in the exhaust purification catalyst.
Performing the heating control is known to cause the following problems. That is, depending on the operation state of the engine, the exhaust temperature is decreased, which deactivates the catalyst. This hampers oxidation of fuel supplied to the catalyst. Continuation of supply of fuel to the exhaust purification catalyst in a deactivated state causes a great amount of fuel to collect on the surface of the catalyst. This in turn increases the amount of accumulated particulate matter. Also, since some of the fuel supplied to the exhaust purification catalyst passes through the catalyst and is emitted, the properties of exhaust gas are degraded.
Not only for burning and removing particulate matter, the heating control is performed, for example, for regenerating a catalyst that has been poisoned with sulfur contained in exhaust gas. When the heating control is performed for releasing sulfur, if the catalyst is deactivated, the sulfur releasing cannot be completed, and thus, the above described problem is caused.